The M40 is issued to US Army and MACVScout kits on certain maps. It has very high damage, low rate of fire, high recoil, good accuracy and a 6X scope meant for long range engagements. Its high damage can kill an enemy in 2 body shots or a single head shot and its accuracy will make it so it can easily hit a target from 200-300 meters away. Although being difficult to use in close-quarters due to its low rate of fire, the small cross hairs and high damage can prove to be effective when no-scoping. The M40 can somewhat be considered an equivalent to the Viet CongM91/30.
Because Battlefield Vietnam features different loadouts for each faction, this list may not be specific enough. Please see each faction's respective page for information on individual weapon kits and loadouts.
In Battlefield: Bad Company 2: Vietnam, the M40 is a weapon issued to the Recon kit. It is a bolt-action sniper rifle capable of one-hit kills at all ranges in hardcore mode, even without magnum ammo, due to its minimum damage being 70%. It is equipped with a 6x scope, but can equip a 12x high powered scope instead. On the left side of the rifle, one can see that it says "Hell Sucks!" scratched in.